Preparation of carbon dioxide acceptors by the melt process



June 23, 1970 G. P. CURRAN ET AL 3,516,808

PREPARATION OF CARBON DIOXIDE ACCEPTORS BY THE MELT PROCESS Filed July19, 1968 TEMP-DEGREES F.

TEMP DEGREES E 2 Shets-Sheet 1 LIQUID MOL C0003 I.O Coo MOL FRACTION cmm ICCIOHIZ //v|//vr0/?5 EVERETT'GOR/N GEORGE F. CUR/PAN WG/ A June 23,1970 G. P. CURRAN ETAL 3,516,808

PREPARATION OF CARBON DIOXIDE ACCEPTORS BY THE MELT PROCESS Filed July19, 1968 2 Sheets-Sheet 2 CALCINED LIMESTONE OR SPENT ACCEPTOR MAKE UPLIMESTONE FEED HOPPER FINES RECYCLE C 800 F.

r v l v INERT BLEED OFF STEAM 1: MELTING UNIT WATER m v n-u "PRlLLlNG"TOWER 0 WATER OUT 34 INERT GAS RECYCL E 40 WITHDRAWAL HOPPER CRUSHINGCALCINI NG REACTIVATED ACCEPTOR TO GASIFICATION PLANT.

INVENTORS EVERETT GORI/V GEORGE P CURRA/V United States Patent r3,516,808 PREPARATION OF CARBON DIOXIDE ACCEPTORS BY THE MELT PROCESSGeorge P. Curran and Everett Gorin, Pittsburgh, Pa., assignors, by mesneassignments, of one-half each to Cousolidation Coal Company, Library,Pa., and the United States of America as represented by the Secretary ofthe Interior Filed July 19, 1968, Ser. No. 746,205 Int. Cl. Ck 1/00,1/20 US. Cl. 48-197 10 Claims ABSTRACT OF THE DISCLOSURE This invention,which relates to the production of CO acceptors, resulted from work doneunder contract with the Office of Coal Research of the US. Department ofthe Interior, and domestic title to the invention is in the government,in accordance with the requirements of the Coal Research Act (30 U.S.C.661-668).

Carbon dioxide (CO) is produced as an undesirable by-product in manyprocesses. In some of these processes, it is desirable to remove the COas fast as it is formed. An example is the steam-carbon reaction whichyields carbon monoxide (CO), methane (CH); and hydrogen (H as well asundesirable CO For the purpose of CO removal, solids which readily reactwith CO may be used directly in the reaction zone, provided they do notreact with the other reactants. Such solids are sometimes called C0acceptors.

Due to its generally high effectiveness and low cost, lime (CaO) is oneof the more favored CO acceptors. During CO removal, the CaOexothermically reacts with CO to form CaCO, which must ultimately bereconverted or regenerated to CaO. Heretofore, such regeneration hasbeen accomplished by heating the carbonate at an elevated temperature toform CaO for recycle and CO gas which is discharged. US. Pat. No.2,705,672 describes such a regeneration procedure.

Our experiments have clearly shown that the lime regenerated in thismanner undergoes progressive decline in its ability to absorb COdespite, or because of, its repeated cycling through the regenerationprocess. The

decline in activity may be attributed to growth in crystallite size ofthe CaO, with consequent progressive reduction in pore volume onrecycling through the CO removal step and associated regeneration step.In any case, the lime finally reaches the point where it is essentiallyinert so far as absorption of CO is concerned.

We have now discovered that such CaO, which has reached a point where itis essentially inert as aCO acceptor, can be converted to highly activeCaO by the following process:

(1) The inactive CaO is converted to Ca( OI-I) by suitable watertreatment, thusly: CaO+H O Ca(OH)- (2) A mixture of the Ca(OH) andeither CaO or CaCO or both is heated to a molten state in the presenceof steam at an elevated pressure sufiiciently high to preventdehydration of the Ca(OH) during melting. A

"ice

eutectic composition is thereby formed between the Ca(OH) and otheringredients.

(3) The eutectic is cooled to a solid state.

(4) The solid eutectic composition (Ca(OH) -CaCO or CaO-Ca(OH) orCaO-Ca(OH) -CaCO as the case may be) is calcined to convert the Ca(OH)to CaO, which is, at this point, an active CO acceptor.

It is therefore an object of the present invention to provide an activeCO acceptor from CaO which has become inert by frequent use andregeneration, or which, in its natural state, is not satisfactorilyactive. Other objects and advantages will be obvious from the detaileddescription of the invention in the following specification taken inconjunction with the drawings wherein:

FIG. 1 is a phase diagram for the system Ca(OH) CaCO FIG. 2. is a phasediagram for system CaO-Ca(OH) FIG. 3 is a flow diagram of a reactivatorplant for spent CO acceptor.

In the practice of the invention inactive lime is first converted toCa(OH) by treatment with H O. This can be done by contacting the limewith high pressure steam (the higher the pressure, the faster thereaction rate) or with water at a temperaure of about 300 to about 400F. and under elevated pressure to maintain the water in a liquid state.

Thereafter, the Ca(OH) is mixed with CaCO and/or CaO. In order tooperate with compositions of the lowest possible melting points, it isobviously desirable to regulate the proportions of the components Ca(OH)CaO, and/ or CaCO; to conform to those of the desired eutectic, althoughour process will still achieve substantial improvements inactivitydespite substantial deviation from the eutectic proportions. Thespecific eutectic compositions for Ca(OI-I) -CaCO and CaO-Ca(OH) areshown in FIGS. 1 and 2, respectively. As for the system CaO- Ca(OH)-CaCO' we have not presently determined the specific proportions of theeutetic composition. However, we have determined that a mixture of thesethree com ponents exists as a liquid over a temperature range of 1000 to1700 F.

When CaCO is to be mixed with the Ca(OH) the lime hydration and mixingsteps can be performed simultaneously. That is, the inactive CaO istreated with steam in the presence of CaCO The mole ratio of Ca(OH) toCaCO; (a mole ratio of 1:1 is preferred) can be adjusted to any desiredvalue during this treatment by adding CO Whatever hydration and mixingtechniques are employed, the mixture of Ca(OH) with CaCO and/or CaO isthen heated to a temperature sufficient to melt the mix, in the presenceof steam under high pressure to prevetn dehydration of the hydroxide.Steam pressures of about 9-30 atmospheres (absolute) are suitable.

The molten mixture is allowed to cool, whereby a eutectic solid isformed. After cooling, the solid is formed into a finely divided state.This can be accomplished by simply allowing the molten mixture tocrystallize to a solid mass which is then ground and screened to recoverthe desired particle size suitable for subsequent use in a gasificationplant wherein CO is removed; or the molten mixture is allowed to dropthrough a cooling tower to form uniformly sized particles. Anothersolidifying arrangement is to spray the melt into an immiscible liquidin which a temperature gradient is maintained such that the liquid dropsfreeze before passing out of the liquid. As an example, the melt couldbe sprayed upwardly through a bath of molten lead.

After solidification, the finely divided composition is subjected to lowtemperature dehydration at about 1000 to -1200--R---and"'atmosphericpressure to convert the" Ca(OH) to CaO, which {resultant material canthen be employed as the CO acceptor. As such, it has been found not onlyto be an active CO acceptor, but'also to be physically'strong soas towithstand many successive regeneration's before a repeat of the processofthis invention must be resorted'to; -'It -is considered to be Withinth'e"fr'ame w'ork of the process of the'present invention'toblend'variousadditives' with the melt. Some additives such as variousother acidic oxides, carbonates, phosphates, including 'by way ofillustration, P B50 SiOg', A1 0 Na CQ act as stabilizers to inhibit CaOcrystal growth. Other'additives act as catalysts for the reactions inwhich the CO acceptors are used.

A generalized type fiowsheet which illustrates how the process can beconducted in practice is shown in FIG. 3. Spent coarse acceptor,attrited acceptor fines, recycle fines from crushing and sizing of themelt, fresh limestone and any combination of the above materials can beemployed as feed in the process. Although the fiowsheet is shown for acontinuous operation, batchwise operation is obviously suitable.

Referring to the figure, a mixture of CaCO (from either spent acceptoror calcined limestone) is present in feed hopper 10. From the hopper thefeed mixture is fed to a melting unit 20 which is a pressurized stirredreactor to which a mixture of steam and CO under pressure at about350-1000 p.s.i.g. is also fed. CaO therein is converted to Ca(OH)- byreaction with steam while a portion is converted to CaCO by reactionwith CO In order to give the desired proportions of Ca(OH) and CaCO inthe melt product, i.e. about 50 mole percent of each, the relativeamounts of steam and CO are appropriately adjusted. If the solid chargein hopper is preheated to about 600800 F., the exothermic heats of thehydration and carbonation reactions are sufficient to achieve thedesired melting temperatures which are of the order of 1200-1400 F.

Melted acceptor is then transferred to a prilling tower 30 through asprayhead device 32. The molten spray is converted to a steady stream ofdroplets of predetermined size suitable for ultimate use as a C0acceptor (e.g., 8 to +28 mesh). By means of a circulation system 34, aninert gas passes upwardly through the tower countercurrent to the moltendroplets. The endothermic reaction of the partial dehydration of 'Ca(OH)is sufficient to freeze or solidify the droplets.

Frozen melt droplets are withdrawn to a hopper 40, and thence to asieving and crushing system 50. Undersize material from crushing andsizing is returned to the melting unit, while oversize material isrecrushed. Desirably sized material is sent to a calcining unit 60 wherethe acceptor is calcined at about 1000-1200 F. and atmospheric pressureto convert the Ca(OH) to CaO before it is sent to a gasification plantwherein the CO is to be removed.

The following examples illustrate the effectiveness of the process ofthe present invention:

EXAMPLE A Fresh South Dakota limestone (=CaCO was calcined to form limeand then split into two portions one portion was marked as Sample A. Theother portion was converted to Ca(OH) by treatment with hot water(liquid) at 300 F. for 1 hour. Thereafter the Ca(OH) was mixed withvarying predetermined amounts of fresh natural CaCO and each mix washeated to molten form in an autoclave in the presence of steam. Thesystems were held at about l-350 and 350 p.s.i.g. for about 20 minutesto-insure complete melting, and then cooled at arate of 5 Foper minuteto just below the CaCO 'Ca(OH) eutectic temperature (1180 F.) and thenrapidly cooled to room temperature.

The resultant solid mass was thencrushed and sized to 14 x 48 mesh, andheated in N at 1600 F. in a fluidized bed reactor. This heatingoperation dehydrated the Ca(OH)- to CaO, and decomposed the CaOO to CaO.Thereafter, the material,.:which*was now all CaO;1.was marked as SamplesB and C.

Reagent grade CaCO and Ca(OI-I) were mixed together in varyingproportions, and the mixe's'were melted, solidified, comminuted andcalcined compltely to CaO in the same manner as that for the naturallimestonederived materials (Samples B and C). Thereafter, the materialwas marked as Samples D, E, F, G and H.

Samples A through H were tested for CO activity by carbonating eachsample with pure CO at one atmosphere (absolute) in a fluidized bed at1500 F. for a predetermined period of time. The following table showsthe results of these activity tests wherein the activity is expressed asthe fractional amount of CaO which is converted to CaCO TAB LE IComposition prior tocalcination(mol It can be seen from Table I that theactivities of melts, as long as they originally contain 40 mole percentor more CaCO (Samples B, C, D, E, and F), are substantially identical tothat obtained from calcined fresh limestone (Sample A).

EXAMPLE B Substantially inactive CO acceptor was prepared by firstpassing fresh South Dakota limestone through the calcining stepdescribed in US. Pat. No. 2,705,672,]and then through the gasificationstep described in said patent. This cyclical calcination-gasificationprocedure was repeated a total of 70 times. After the final gasificationstep, the CaCO -containing material was calcined to form CaO and thensplit into two portions. One portion (Sample I) was tested for activityin the manner set forth in Example A. The other portion was firsttreated with hot water at 300 F. for one hour to produce Ca(OH)Thereafter, CaCO -Ca(OH) -CaO was formed therefrom by mixing togetherthe appropriate ingedients, melting the mixture in the manner previouslyset forth in Example A, solidify ing the melt, and calcining theresultant solid materials. These latter samples, which were markedSamples J and K, were then tested for activity'in the manner set forthinExampleA. F

I The following table shows the results of testing Samples LJandK' s,

TAB LE II percent) V I p CaCOa Ca(0H)2 0110 Activity Deactivated CaO,Sample,-I -:.i 3 "I 16 Celcined melt from deactivated '1' I I CaO:

Sample I 17 72 11 ."58 Sample K .18 78 v 9 54 1 It can be seen fromTable II that the activity of the acceptor has increasedthree fold afterbeing treated by the process of the present invention. Further, bycomparing Table II with TableI, it can be seen that the activity of theregenerated acceptor is essentially indistinguishable from meltsprepared from fresh materials. It is likely that the additionof CO tothe regenerated acceptor to increase the CaCO content'thereof (prior tomelting and subsequent calcination) could further increase the activityof the regenerated acceptor until it would be substantially equal to theactivity of calcined fresh limestone.

EXAMPLE Ca(OH) was prepared by calcining and then hydrating naturalSouth Dakota limestone in the manner set forth in the example A.Thereafter it was mixed with natural limestone and heated to 1350" C.under 350 p.s.i.g. steam pressure until it was completely melted. Aftersolidification in the manner set forth in example A, crushing and sizingwere employed to obtain 16 x 28 mesh particles containing 50 molepercent Ca(OH) and 50 mole percent CaCO Ca(OH): therein was thenconverted to CaO by heating to about 1000 C. at atmospheric pressure.This melt-derived acceptor material was then passed through theregeneration procedure described in US. Pat. No. 2,705,672 to completelyconvert it to CaO, and then passed through the gasification proceduredescribed in said patent. Fresh South Dakota limestone (16 x 28 mesh)was also serially passed through the regeneration and gasificationprocedures described in said patent.

Table III below shows the operating conditions employed with theseacceptors during gasification of 35 x 150 mesh devolatilized lignitechar.

TABLE III Calcined Meltlimestone derived acceptor acceptor General:

Gasification, regeneration pressure Acceptor circulation rate (lb/hr.)5. 23 6. 53 Gasifier Conditions:

Temperature, F- 1, 500 1, 500 Char teed rate (lb./hr.) 1. 6 1. 6 Charbed inventory, 1b.. 7. 8 7. 8 Fluidizing velocity, ft./sec 0.27 0.27Outlet gas composition from gasifier, vol. percent:

H2 20 20 H2" 38 38 CH4 5 5 CO 10 C02. 6 6 N2 21 21 Regeneratorconditions:

Temperature, F 1, 910 1, 940 Fluidizing Velocity it./sec 1. 1 1. 1Outlet gas composition from regenerator, vol.

percent:

1 20 atm. absolute.

The operation simulates, in all details, a commercial CO acceptor systemwith the exception that the heat of calcination in the regenerator wassupplied electrically instead of by in situ char combustion.

During the operation of the continuous unit, the acceptor enters thegasifier in the fully calcined condition and is recarbonated on fallingthroughthe char bed. The driving force for recarbonation, i.e. Pco -(Pcoequil., is about 0.5-0.8 atmospheres and the residence time of theacceptor is about 7 minutes.

This cyclical calcination-gasification procedure was repeated many timesfor each material. The activity of each acceptor, measured as the molesCaCO in the acceptor leaving the gasifier per total moles Ca in theacceptor leaving 'the' gasifier, was periodically measured, and thefollowing results were obtained:

TABLE IV Activity Fresh llmestone- Melt-derived From Table IV it can beseen that the initial activities of the limestone and the melt acceptorare substantially identical. The melt declines more rapidly on cyclingthrough the system. However, it should be noted from Table III that theregenerator temperature was about 30 F. higher in the melt acceptor runand this may be the cause of the more rapid decline in activity.

The higher temperature in the melt acceptor run was not accidental. Theregenerator temperature was limited to 1910 F. in the limestone run toprevent agglomeration in the regenerator. The melt acceptor showed lesstendency to agglomerate, which permitted an increase in regeneratortemperature to 1940 F.

The run also demonstrated one of the other beneficial features of themelt acceptor, i.e. its high physical strength The attrition rate onpassing the acceptor through the process was extremely small, i.e. muchless than 0.5 weight percent/ cycle.

While the process is well adapted to carry out the objects of thepresent invention, it is to be understood that various modifications andchanges may be made all coming within the scope of the following claims.

What is claimed is:

1. A process for treating lime comprising (a) converting said lime toCa(OH) and forming a mixture of said Ca(OH); with CaCO and/or CaO;

(b) heating said mixture in the presence of steam until said mixture ismelted, said heating being conducted at an elevated pressure which issufficiently high to prevent dehydration of said Ca(OH) whereby a Ca(OH)-containing melt is formed;

(0) allowing said melt to cool to a Ca(OH) -containing solid statematerial; and

(d) calcining said solid material to convert said Ca(OH) therein to CaOwhich is, at this point, an active CO acceptor.

2. The process of claim 1 wherein said lime, prior to step (a), isemployed as a C0 acceptor in a CO -producing process to thereby formCaCO which CaCO is thereafter calcined to regenerate CaO for reuse insaid CO producing process; and wherein said lime has become inert orless eifective as a C0 acceptor after repeated calcination and reuse asa C0 acceptor in said cO -producing process.

3. The process of claim 2 in which said lime is converted to Ca(OH); instep (a) of claim 1 by treatment of said lime with H 0.

4. The process of claim 2 in which said coolingstep (c) of claim 1 isaccomplished by passing said melted mixture through a prilling tower toform solid droplets.

5. The process of claim 2 wherein said CO -producing process is asteam-carbon reaction process in which CO, CH H and C0,, are produced.

6. The process of claim 2 wherein the pressure during melting of saidmixture is about 9-30 atmospheres absolute. v

7. The process of claim 2 wherein said conversion and mixing step (a)and melting step (b) of claim 1 are carried out by (a) mixing said limewith CaCO in a pressurized reactor; and (b) feeding a mixture of steamand CO to said reactor while maintaining the pressure therein betweenabout 350 and 1000 p.s.i.g. and the temperature between about 1200-1400F., whereby part of said lime is converted to Ca(OI-I) by reaction withsaid steam, and the remaining part is converted to CaCO by reaction withsaid CO2, the relative amounts of steam and CO reacted being adjusted toyield a melt containing about 50 mole percent of Ca(OH) and 50 molepercent of CaCO,,. 8. The process of claim 2 in which said melt containsCa(OH) and CaCO in approximately 1:1 molar ratio. 9. The process ofclaim 8 wherein said steps (a) and (b) of claim 1 include mixing saidlime with CaCO in the presence of steam and C0 10. The process of claim9 in which said CO -producing process is a steam-carbon reaction processin which said CO, CH H and C0; are produced.

References Cited UNITED STATES PATENTS MORRIS O. WOLK, Primary ExaminerR. E. SERWIN, Assistant Examiner US. Cl. X.R.

